Jokir, have you thought about making sugar cookies and decorating them with green candied sugar sprinkled over the tops of them? Also, what about a white cake shaped like a shamrock, iced with white icing and bordered along the edge with green icing?
I made a shamrock chocolate cake using 3 heart-shaped pans, put the cake on a green foil-covered pastry board and iced it with green coloured icing. I made my cake from scratch, but I believe that a mix would serve the purpose. I saw a teacher do here when I was teaching was make the cookies using the shamrock-shaped cookie cutter and decorated them as described above. Another thing I saw were small cupcakes with white icing on the top of them garnished with a green jelly bean. Also, if someone else is paying for it, have you thought of getting small zip lock bags and putting some green and white jelly beans in it to give the children? These are just a few things which I have seen done here when I was teaching school. Coming from Canada's most Irish city, no doubt there are a few more things that they do. I'll have to do a bit of research. By all means, please follow some of the other suggestions Targirl mentioned. There's a recipe for sugar cookies on the site here to help you out.